Portable coordinate measuring systems are well known and play an increasingly important role in science and industry. Both mechanical and electro-optical approaches are known, but for the large measurement volumes often encountered, e.g., in automotive and aerospace applications, electro-optical systems dominate. Two important types of such systems are laser tracker systems and real-time photogrammetry-based systems.
Known laser tracker systems (see, e.g., FIG. 1) are based on a device 10 such as an interferometer or laser rangefinder which is capable of measuring the distance (d) to a reflective object element 12 (such as, e.g., a measurement mirror or prism), using a mechanism 14 (such as, e.g., a mirror) for directing a laser beam from the distance measuring device 10 in the desired direction, combined with a means 16 for determining this direction. A distance and two angles are measured for the interferometer laser beam in order to calculate the spatial coordinates of the reflection point or center of the measurement mirror or prism 12. The various components of a typical laser tracker system may be housed in a unit 18. Laser tracking systems are known in a number of different embodiments and are described, for example, in U.S. Pat. No. 4,714,339.
Commercially available laser tracking systems include those using spherically mounted retro-reflectors—SMRs—as the reflective object element 12. A more recent development is systems using an absolute distance meter, commonly known as an ADM, as the distance measuring device 10. Other systems use what is commonly referred to as a laser radar, and these systems use a reflective object element 12 which is not a separate piece of hardware, but which is the surface or a surface element of the object to measured itself. In other words, the reflective object element 12 (e.g., measurement mirror or prism) is replaced and the beam is reflected off the object surface itself. For such systems, certain restrictions exist on the permissible reflectivity of the object surface.
Current laser tracker and similar systems can only acquire the coordinates of a single point at any moment in time. This limitation means that only the position of the center of a hand-held reflective object element (e.g., measurement mirror or prism) can be determined, while the orientation or rotation angles of the reflective object element (measurement mirror or prism) remain unknown. Therefore, using current laser tracker systems, it is not possible to measure points other than those directly visible from the system main unit 18 comprising the distance measuring device 10. Also, using current laser tracker systems, it is difficult to measure small details, features or shapes that do not lend themselves to touching with the relatively bulky assemblies of the reflective object element 12. Attempts have been made to overcome these limitations. For example, U.S. Pat. No. 5,530,549 (see, e.g., FIG. 2) describes an optical means that allows a limited-length offset mechanism (such as a probe 20) to replace the reflective object element (such as a measurement mirror or prism 22), which is positioned exactly opposite the measurement mirror or prism relative to an absolutely flat mirror 23. Thereby, the laser tracker determines the coordinates of the virtual reflection center 21 although the laser beam B is actually reflected at the center of the reflective object element 22.
This development is suitable for a range of simple measurement tasks such as the measurement of small-diameter holes in planar surfaces, but it is limited with regard to freedom of orientation, length of probe and shape of probe. The mirror arrangement further relies on mechanical precision since no mathematical compensation for manufacturing inaccuracies—or subsequent dimensional changes often encountered during industrial use—can be made.
The second type of know systems, real-time photogrammetry-based systems use one or more cameras to observe significant features—often targets—in the measurement scene and then calculate the targets' spatial coordinates based on techniques ranging from simple triangulation methods to advanced bundle adjustment calculations. A number of embodiments of such systems are known, e.g., DE 41,24,174, EP 607,303 and U.S. Pat. No. 6,389,158, owned by Metronor AS of Norway.
A particular benefit of photogrammetry-based systems is their ability to measure the coordinates of multiple points simultaneously. This has enabled the development of flexible probing systems such as those described, e.g., in SE 456,454, whereby probes of any geometry can be attached to a measurement handle that has multiple targets embedded. Once the position and orientation of the measurement handle has been determined, the spatial coordinates of the probe—and therefore the point to be measured—can be computed.
Compared with laser tracker systems, photogrammetry-based systems provide for relatively efficient measurement of the small, complex features and details often found in industrial products, and they provide for efficient measurement of points outside direct line of sight.
Systems that combine features and elements of photogrammetry-based systems and laser tracker systems and that try to provide the benefits of both types of systems are known. For example, U.S. Pat. No. 6,166,809 (owned by Metronor AS of Norway) and EP 880,674, respectively, describe a combined system whereby the probe consists of a laser tracker measurement mirror or prism combined with a measurement handle of a real-time photogrammetry system. This system addresses many of the shortcomings of simple laser tracker systems such as the one described in U.S. Pat. No. 5,530,549, for instance, but is complex and expensive. Also, the accuracy with which the probe orientation or rotations can be determined depends on a number of factors including distance and camera resolution and is a limitation.
It is an objective of the present invention to provide a system and method for coordinate measuring of low complexity and high performance. This objective is attained by the systems and methods described herein and set forth in the appended claims.